Unraveling the evolution of auxin signaling

Structure and Function of Auxin Transporters

Auxins, a group of central hormones in plant growth and development, are transported by a diverse range of transporters with distinct biochemical and structural properties. This review summarizes the current knowledge on all known auxin transporters with respect to their biochemical and biophysical properties and the methods used to characterize them. In particular, we focus on the recent advances that were made concerning the PIN-FORMED family of auxin exporters. Insights derived from solving their structures have improved our understanding of the auxin export process, and we discuss the current state of the art on PIN-mediated auxin transport, including the use of biophysical methods to examine their properties. Understanding the mechanisms of auxin transport is crucial for understanding plant growth and development, as well as for the development of more effective strategies for crop production and plant biotechnology.

Heterotrophic bacteria trigger transcriptome remodelling in the photosynthetic picoeukaryote Micromonas commoda

Marine biogeochemical cycles are built on interactions between surface ocean microbes, particularly those connecting phytoplankton primary producers to heterotrophic bacteria. Details of these associations are not well understood, especially in the case of direct influences of bacteria on phytoplankton physiology. Here we catalogue how the presence of three marine bacteria (Ruegeria pomeroyi DSS-3, Stenotrophomonas sp. SKA14 and Polaribacter dokdonensis MED152) individually and uniquely impact gene expression of the picoeukaryotic alga Micromonas commoda RCC 299. We find a dramatic transcriptomic remodelling by M. commoda after 8 h in co-culture, followed by an increase in cell numbers by 56 h compared with the axenic cultures. Some aspects of the algal transcriptomic response are conserved across all three bacterial co-cultures, including an unexpected reduction in relative expression of photosynthesis and carbon fixation pathways. Expression differences restricted to a single bacterium are also observed, with the Flavobacteriia P. dokdonensis uniquely eliciting changes in relative expression of algal genes involved in biotin biosynthesis and the acquisition and assimilation of nitrogen. This study reveals that M. commoda has rapid and extensive responses to heterotrophic bacteria in ways that are generalizable, as well as in a taxon specific manner, with implications for the diversity of phytoplankton-bacteria interactions ongoing in the surface ocean.

TOPLESS Corepressors as an Emerging Hub of Plant Pathogen Effectors

Transcriptional corepressors form an ancient and essential layer of gene expression control in eukaryotes. TOPLESS and TOPLESS-RELATED (TPL/TPR) proteins constitute a conserved family of Groucho (Gro)/thymidine uptake 1 (Tup1)-type transcriptional corepressors and control diverse growth, developmental, and stress signaling responses in plants. Because of their central and versatile regulatory roles, they act as a signaling hub to integrate various input signaling pathways in the transcriptional responses. Recently, increasing pieces of evidence indicate the roles of TPL/TPR family proteins in the modulation of plant immunity. This is supported by studies on effectors of distantly related pathogens that target TPL/TPR proteins in planta. In this short review, we will summarize the latest findings concerning pathogens targeting plant TPL/TPR proteins to manipulate plant signaling responses for the successful invasion of their hosts. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The efficient capture of polysaccharides in Tetradesmus obliquus of indole-3-acetic acid coupling sludge extraction

Polysaccharide is an important biomass of algae. The sludge extract is rich in organic substances, which can be used by algae for biomass growth and high-value biomass synthesis, but its organic toxicity has an inhibitory effect on algae. To overcome inhibition and improve polysaccharide enrichment, Tetradesmus obliquus was cultured with sludge extract with different indole-3-acetic acid (IAA) concentrations. Within 30 days of the culture cycle, T. obliquus showed in good condition at the IAA dosage content of 10-6 M, the maximum cell density and dry weight were respectively (106.78 ± 2.20) × 106 cell/mL and 2.941 ± 0.067 g/L while the contents of chlorophyll-a, chlorophyll-b, and carotenoid were 1.79, 1.91 and 2.80 times that of the blank group, respectively. The highest polysaccharide accumulation was obtained under this culture condition, reaching 533.15 ± 21.11 mg/L on the 30th day, which was 2.49 times that in the blank group. By FT-IR and NMR analysis, it was found that the polysaccharides of T. obliquus were sulfated polysaccharide with glucose and rhamnose as the main monosaccharides. Proteomic showed that the up-regulation of A0A383WL26 and A0A383WLM8 enhanced the light trapping ability, and A0A383WMJ2 enhanced the accumulation of NADPH. The up-regulation of A0A383WHD5 and A0A383WAY6 indicated that IAA culture could repair the damage caused by sludge toxicity, thus promoting the accumulation of biomass. The above findings provided new insights into the mechanism of sludge toxicity removal of T. obliquus and the enhancement of the polysaccharide accumulation effect under different concentrations of IAA.

Genetic evidence for algal auxin production in Chlamydomonas and its role in algal-bacterial mutualism

Interactions between algae and bacteria are ubiquitous and play fundamental roles in nutrient cycling and biomass production. Recent studies have shown that the plant auxin indole acetic acid (IAA) can mediate chemical crosstalk between algae and bacteria, resembling its role in plant-bacterial associations. Here, we report a mechanism for algal extracellular IAA production from L-tryptophan mediated by the enzyme L-amino acid oxidase (LAO1) in the model Chlamydomonas reinhardtii. High levels of IAA inhibit algal cell multiplication and chlorophyll degradation, and these inhibitory effects can be relieved by the presence of the plant-growth-promoting bacterium (PGPB) Methylobacterium aquaticum, whose growth is mutualistically enhanced by the presence of the alga. These findings reveal a complex interplay of microbial auxin production and degradation by algal-bacterial consortia and draws attention to potential ecophysiological roles of terrestrial microalgae and PGPB in association with land plants.

The origin and early evolution of cytokinin signaling

Angiosperms, especially Arabidopsis and rice, have long been at the center of plant research. However, technological advances in sequencing have led to a dramatic increase in genome and transcriptome data availability across land plants and, more recently, among green algae. These data allowed for an in-depth study of the evolution of different protein families - including those involved in the metabolism and signaling of phytohormones. While most early studies on phytohormone evolution were phylogenetic, those studies have started to be complemented by genetic and biochemical studies in recent years. Examples of such functional analyses focused on ethylene, jasmonic acid, abscisic acid, and auxin. These data have been summarized recently. In this review, we will focus on the progress in our understanding of cytokinin biology. We will use these data to synthesize key points about the evolution of cytokinin metabolism and signaling, which might apply to the evolution of other phytohormones as well.

The differential binding and biological efficacy of auxin herbicides

BACKGROUND

Auxin herbicides have been used for selective weed control for 75 years and they continue to be amongst the most widely used weed control agents globally. The auxin herbicides fall into five chemical classes, with two herbicides not classified, and in all cases it is anticipated that recognition in the plant starts with binding to the Transport Inhibitor Response 1 (TIR1) family of auxin receptors. There is evidence that some classes of auxins act selectively with certain clades of receptors, although a comprehensive structure-activity relationship has not been available.

RESULTS

Using purified receptor proteins to measure binding efficacy we have conducted quantitative structure activity relationship (qSAR) assays using representative members of the three receptor clades in Arabidopsis, TIR1, AFB2 and AFB5. Complementary qSAR data for biological efficacy at the whole-plant level using root growth inhibition and foliar phytotoxicity assays have also been analyzed for each family of auxin herbicides, including for the afb5-1 receptor mutant line.

CONCLUSIONS

Comparisons of all these assays highlight differences in receptor selectivity and some systematic differences between results for binding in vitro and activity in vivo. The results could provide insights into weed spectrum differences between the different classes of auxin herbicides, as well as the potential resistance and cross-resistance implications for this herbicide class. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Auxin Crosstalk with Reactive Oxygen and Nitrogen Species in Plant Development and Abiotic Stress

The phytohormone auxin acts as an important signaling molecule having regulatory functions during the growth and development of plants. Reactive oxygen species (ROS) are also known to perform signaling functions at low concentrations; however, over-accumulation of ROS due to various environmental stresses damages the biomolecules and cell structures and leads to cell death, and therefore, it can be said that ROS act as a double-edged sword. Nitric oxide (NO), a gaseous signaling molecule, performs a wide range of favorable roles in plants. NO displays its positive role in photomorphogenesis, root growth, leaf expansion, seed germination, stomatal closure, senescence, fruit maturation, mitochondrial activity and metabolism of iron. Studies have revealed the early existence of these crucial molecules during evolution. Moreover, auxin, ROS and NO together show their involvement in various developmental processes and abiotic stress tolerance. Redox signaling is a primary response during exposure of plants to stresses and shows a link with auxin signaling. This review provides updated information related to crosstalk between auxin, ROS and NO starting from their evolution during early Earth periods and their interaction in plant growth and developmental processes as well as in the case of abiotic stresses to plants.

Identification of a major QTL, Parth6.1 associated with parthenocarpic fruit development in slicing cucumber genotype, Pusa Parthenocarpic Cucumber-6

Parthenocarpy is an extremely important trait that revolutionized the worldwide cultivation of cucumber under protected conditions. Pusa Parthenocarpic Cucumber-6 (PPC-6) is one of the important commercially cultivated varieties under protected conditions in India. Understanding the genetics of parthenocarpy, molecular mapping and the development of molecular markers closely associated with the trait will facilitate the introgression of parthenocarpic traits into non-conventional germplasm and elite varieties. The F1, F2 and back-crosses progenies with a non-parthenocarpic genotype, Pusa Uday indicated a single incomplete dominant gene controlling parthenocarpy in PPC-6. QTL-seq comprising of the early parthenocarpy and non-parthenocarpic bulks along with the parental lines identified two major genomic regions, one each in chromosome 3 and chromosome 6 spanning over a region of 2.7 Mb and 7.8 Mb, respectively. Conventional mapping using F2:3 population also identified two QTLs, Parth6.1 and Parth6.2 in chromosome 6 which indicated the presence of a major effect QTL in chromosome 6 determining parthenocarpy in PPC-6. The flanking markers, SSR01148 and SSR 01012 for Parth6.1 locus and SSR10476 and SSR 19174 for Parth6.2 locus were identified and can be used for introgression of parthenocarpy through the marker-assisted back-crossing programme. Functional annotation of the QTL-region identified two major genes, Csa_6G396640 and Csa_6G405890 designated as probable indole-3-pyruvate monooxygenase YUCCA11 and Auxin response factor 16, respectively associated with auxin biosynthesis as potential candidate genes. Csa_6G396640 showed only one insertion at position 2179 in the non-parthenocarpic parent. In the case of Csa_6G405890, more variations were observed between the two parents in the form of SNPs and InDels. The study provides insight about genomic regions, closely associated markers and possible candidate genes associated with parthenocarpy in PPC-6 which will be instrumental for functional genomics study and better understanding of parthenocarpy in cucumber.

How was apical growth regulated in the ancestral land plant? Insights from the development of non-seed plants

Land plant life cycles are separated into distinct haploid gametophyte and diploid sporophyte stages. Indeterminate apical growth evolved independently in bryophyte (moss, liverwort, and hornwort) and fern gametophytes, and tracheophyte (vascular plant) sporophytes. The extent to which apical growth in tracheophytes co-opted conserved gametophytic gene networks, or exploited ancestral sporophytic networks, is a long-standing question in plant evolution. The recent phylogenetic confirmation of bryophytes and tracheophytes as sister groups has led to a reassessment of the nature of the ancestral land plant. Here, we review developmental genetic studies of apical regulators and speculate on their likely evolutionary history.

Into a dilemma of plants: the antagonism between chemical defenses and growth

Chemical defenses are imperative for plant survival, but their production is often associated with growth restrictions. Here we review the most recent theories to explain this complex dilemma of plants. Plants are a nutritional source for a myriad of pests and pathogens that depend on green tissues to complete their life cycle. Rather than remaining passive victims, plants utilize an arsenal of chemical defenses to fend off biotic attack. While the deployment of such barriers is imperative for survival, the production of these chemical defenses is typically associated with negative impacts on plant growth. Here we discuss the most recent theories which explain this highly dynamic growth versus defense dilemma. Firstly, we discuss the hypothesis that the antagonism between the accumulation of chemical defenses and growth is rooted in the evolutionary history of plants and may be a consequence of terrestrialization. Then, we revise the different paradigms available to explain the growth versus chemical defense antagonism, including recent findings that update these into more comprehensive and plausible theories. Finally, we highlight state-of-the-art strategies that are now allowing the activation of growth and the concomitant production of chemical barriers in plants. Growth versus chemical defense antagonism imposes large ecological and economic costs, including increased crop susceptibility to pests and pathogens. In a world where these plant enemies are the main problem to increase food production, we believe that this review will summarize valuable information for future studies aiming to breed highly defensive plants without the typical accompanying penalties to growth.

Building an embryo: An auxin gene toolkit for zygotic and somatic embryogenesis in Brazilian pine

Many studies in the model species Arabidopsis thaliana characterized genes involved in embryo formation. However, much remains to be learned about the portfolio of genes that are involved in signal transduction and transcriptional regulation during plant embryo development in other species, particularly in an evolutionary context, especially considering that some genes involved in embryo patterning are not exclusive of land plants. This study, used a combination of domain architecture phylostratigraphy and phylogenetic reconstruction to investigate the evolutionary history of embryo patterning and auxin metabolism (EPAM) genes in Viridiplantae. This approach shed light on the co-optation of auxin metabolism and other molecular mechanisms that contributed to the radiation of land plants, and specifically to embryo formation. These results have potential to assist conservation programs, by directing the development of tools for obtaining somatic embryos. In this context, we employed this methodology with critically endangered and non-model species Araucaria angustifolia, the Brazilian pine, which is current focus of conservation efforts using somatic embryogenesis. So far, this approach had little success since somatic embryos fail to completely develop. By profiling the expression of genes that we identified as necessary for the emergence of land-plant embryos, we found striking differences between zygotic and somatic embryos that might explain the developmental arrest and be used to improve A. angustifolia somatic culture.

A comparative whole-genome approach identifies bacterial traits for marine microbial interactions

Microbial interactions shape the structure and function of microbial communities with profound consequences for biogeochemical cycles and ecosystem health. Yet, most interaction mechanisms are studied only in model systems and their prevalence is unknown. To systematically explore the functional and interaction potential of sequenced marine bacteria, we developed a trait-based approach, and applied it to 473 complete genomes (248 genera), representing a substantial fraction of marine microbial communities. We identified genome functional clusters (GFCs) which group bacterial taxa with common ecology and life history. Most GFCs revealed unique combinations of interaction traits, including the production of siderophores (10% of genomes), phytohormones (3-8%) and different B vitamins (57-70%). Specific GFCs, comprising Alpha- and Gammaproteobacteria, displayed more interaction traits than expected by chance, and are thus predicted to preferentially interact synergistically and/or antagonistically with bacteria and phytoplankton. Linked trait clusters (LTCs) identify traits that may have evolved to act together (e.g., secretion systems, nitrogen metabolism regulation and B vitamin transporters), providing testable hypotheses for complex mechanisms of microbial interactions. Our approach translates multidimensional genomic information into an atlas of marine bacteria and their putative functions, relevant for understanding the fundamental rules that govern community assembly and dynamics.

Auxin's origin: do PILS hold the key?

Auxin is a key regulator of many developmental processes in land plants and plays a strikingly similar role in the phylogenetically distant brown seaweeds. Emerging evidence shows that the PIN and PIN-like (PILS) auxin transporter families have preceded the evolution of the canonical auxin response pathway. A wide conservation of PILS-mediated auxin transport, together with reports of auxin function in unicellular algae, would suggest that auxin function preceded the advent of multicellularity. We find that PIN and PILS transporters form two eukaryotic subfamilies within a larger bacterial family. We argue that future functional characterisation of algal PIN and PILS transporters can shed light on a common origin of an auxin function followed by independent co-option in a multicellular context.

Auxin Transporters-A Biochemical View

From embryogenesis to fruit formation, almost every aspect of plant development and differentiation is controlled by the cellular accumulation or depletion of auxin from cells and tissues. The respective auxin maxima and minima are generated by cell-to-cell auxin transport via transporter proteins. Differential auxin accumulation as a result of such transport processes dynamically regulates auxin distribution during differentiation. In this review, we introduce all auxin transporter (families) identified to date and discuss the knowledge on prominent family members, namely, the PIN-FORMED exporters, ATP-binding cassette B (ABCB)-type transporters, and AUX1/LAX importers. We then concentrate on the biochemical features of these transporters and their regulation by posttranslational modifications and interactors.

The bryophytes Physcomitrium patens and Marchantia polymorpha as model systems for studying evolutionary cell and developmental biology in plants

Bryophytes are nonvascular spore-forming plants. Unlike in flowering plants, the gametophyte (haploid) generation of bryophytes dominates the sporophyte (diploid) generation. A comparison of bryophytes with flowering plants allows us to answer some fundamental questions raised in evolutionary cell and developmental biology. The moss Physcomitrium patens was the first bryophyte with a sequenced genome. Many cell and developmental studies have been conducted in this species using gene targeting by homologous recombination. The liverwort Marchantia polymorpha has recently emerged as an excellent model system with low genomic redundancy in most of its regulatory pathways. With the development of molecular genetic tools such as efficient genome editing, both P. patens and M. polymorpha have provided many valuable insights. Here, we review these advances with a special focus on polarity formation at the cell and tissue levels. We examine current knowledge regarding the cellular mechanisms of polarized cell elongation and cell division, including symmetric and asymmetric cell division. We also examine the role of polar auxin transport in mosses and liverworts. Finally, we discuss the future of evolutionary cell and developmental biological studies in plants.

Specialized metabolism and development: An unexpected friendship

Plants produce a myriad of metabolites. Some of them have been regarded for a long time as secondary or specialized metabolites and are considered to have functions mostly in defense and the adaptation of plants to their environment. However, in the last years, new research has shown that these metabolites can also have roles in the regulation of plant growth and development, some acting as signals, through the interaction with hormonal pathways, and some independently of them. These reports provide a glimpse of the functional possibilities that specialized metabolites present in the modulation of plant development and encourage more research in this direction.

Evolution and diversity of the angiosperm anther: trends in function and development

Anther development and dehiscence is considered from an evolutionary perspective to identify drivers for differentiation, functional conservation and to identify key questions for future male reproduction research. Development of viable pollen and its timely release from the anther are essential for fertilisation of angiosperm flowers. The formation and subsequent dehiscence of the anther are under tight regulatory control, and these processes are remarkably conserved throughout the diverse families of the angiosperm clade. Anther development is a complex process, which requires timely formation and communication between the multiple somatic anther cell layers (the epidermis, endothecium, middle layer and tapetum) and the developing pollen. These layers go through regulated development and selective degeneration to facilitate the formation and ultimate release of the pollen grains. Insight into the evolution and divergence of anther development and dehiscence, especially between monocots and dicots, is driving greater understanding of the male reproductive process and increased, resilient crop yields. This review focuses on anther structure from an evolutionary perspective by highlighting their diversity across plant species. We summarise new findings that illustrate the complexities of anther development and evaluate how they challenge established models of anther form and function, and how they may help to deliver future sustainable crop yields.

Repressor for hire! The vital roles of TOPLESS-mediated transcriptional repression in plants

Transcriptional corepressors play important roles in establishing the appropriate levels of gene expression during growth and development. The TOPLESS (TPL) family of corepressors are critical for all plant life. TPLs are involved in numerous developmental processes and in the response to extrinsic challenges. As such these proteins have been the focus of intense study since Long and colleagues first described the TPL corepressor in 2006. In this review we will explore the evolutionary history of these essential plant-specific proteins, their mechanism of action based on recent structural analyses, and the myriad of pathways in which they function. We speculate how relatively minor changes in the peptide sequence of transcriptional regulators allowed them to recruit TPL into new processes, driving innovation and resulting in TPL becoming vital for plant development.

On the Evolutionary Origins of Land Plant Auxin Biology

Indole-3-acetic acid, that is, auxin, is a molecule found in a broad phylogenetic distribution of organisms, from bacteria to eukaryotes. In the ancestral land plant auxin was co-opted to be the paramount phytohormone mediating tropic responses and acting as a facilitator of developmental decisions throughout the life cycle. The evolutionary origins of land plant auxin biology genes can now be traced with reasonable clarity. Genes encoding the two enzymes of the land plant auxin biosynthetic pathway arose in the ancestral land plant by a combination of horizontal gene transfer from bacteria and possible neofunctionalization following gene duplication. Components of the auxin transcriptional signaling network have their origins in ancestral alga genes, with gene duplication and neofunctionalization of key domains allowing integration of a portion of the preexisting transcriptional network with auxin. Knowledge of the roles of orthologous genes in extant charophycean algae is lacking, but could illuminate the ancestral functions of both auxin and the co-opted transcriptional network.

The production of auxin by dying cells

In this review, I discuss the possibility that dying cells produce much of the auxin in vascular plants. The natural auxin, indole-3-acetic acid (IAA), is derived from tryptophan by a two-step pathway via indole pyruvic acid. The first enzymes in the pathway, tryptophan aminotransferases, have a low affinity for tryptophan and break it down only when tryptophan levels rise far above normal intracellular concentrations. Such increases occur when tryptophan is released from proteins by hydrolytic enzymes as cells autolyse and die. Many sites of auxin production are in and around dying cells: in differentiating tracheary elements; in root cap cells; in nutritive tissues that break down in developing flowers and seeds; in senescent leaves; and in wounds. Living cells also produce auxin, such as those transformed genetically by the crown gall pathogen. IAA may first have served as an exogenous indicator of the presence of nutrient-rich decomposing organic matter, stimulating the production of rhizoids in bryophytes. As cell death was internalized in bryophytes and in vascular plants, IAA may have taken on a new role as an endogenous hormone.

Uncharted routes: exploring the relevance of auxin movement via plasmodesmata

Auxin is an endogenous small molecule with an incredibly large impact on growth and development in plants. Movement of auxin between cells, due to its negative charge at most physiological pHs, strongly relies on families of active transporters. These proteins import auxin from the extracellular space or export it into the same. Mutations in these components have profound impacts on biological processes. Another transport route available to auxin, once the substance is inside the cell, are plasmodesmata connections. These small channels connect the cytoplasms of neighbouring plant cells and enable flow between them. Interestingly, the biological significance of this latter mode of transport is only recently starting to emerge with examples from roots, hypocotyls and leaves. The existence of two transport systems provides opportunities for reciprocal cross-regulation. Indeed, auxin levels influence proteins controlling plasmodesmata permeability, while cell-cell communication affects auxin biosynthesis and transport. In an evolutionary context, transporter driven cell-cell auxin movement and plasmodesmata seem to have evolved around the same time in the green lineage. This highlights a co-existence from early on and a likely functional specificity of the systems. Exploring more situations where auxin movement via plasmodesmata has relevance for plant growth and development, and clarifying the regulation of such transport, will be key aspects in coming years.This article has an associated Future Leader to Watch interview with the author of the paper.

Leaf dorsoventrality candidate gene CpARF4 has conserved expression pattern but divergent tasiR-ARF regulation in the water fern Ceratopteris pteridoides

PREMISE

Leaves are traditionally classified into microphylls and megaphylls, and recently have been regarded as independently originating in lycophytes, ferns, and seed plants. The developmental genetics of leaf dorsoventrality, a synapomorphy in vascular plants, has been extensively studied in flowering plants. AUXIN RESPONSE FACTOR4 (ARF4) genes are key to leaf abaxial identity in flowering plants, but whether they exist in ferns is still an open question.

METHODS

ARF4 genes from Ceratopteris pteridoides, Cyrtomium guizhouense, and Parathelypteris nipponica were mined from transcriptomes and investigated in terms of evolutionary phylogeny and sequence motifs, with a focus on the tasiR-ARF binding site. In situ hybridization was used to localize expression of CpARF4 in Ceratopteris pteridoides. 5'RNA ligase-mediated-RACE was employed to verify whether CpARF4 transcripts were sliced by tasiR-ARF.

RESULTS

ARF4 genes exist in ferns, and this lineage originates from a gene duplication in the common ancestor of ferns and seed plants. ARF4 genes are of a single copy in the ferns studied here, and they contain divergent and, at most, one tasiR-ARF binding site. CpARF4 is expressed in the abaxial but not the adaxial domain of leaf primordia at various developmental stages. Transcript slicing guided by tasiR-ARF is active in C. pteridoides, but CpARF4 probably has not been affected by it.

CONCLUSIONS

Fern ARF4 genes differ in copy number and tasiR-ARF regulation relative to flowering plants, though they can be similarly expressed in the abaxial domain of leaves, revealing a key role for ARF4 genes in the evolution of leaf dorsoventrality of vascular plants.

The Phylogeny of Class B Flavoprotein Monooxygenases and the Origin of the YUCCA Protein Family

YUCCA (YUCCA flavin-dependent monooxygenase) is one of the two enzymes of the main auxin biosynthesis pathway (tryptophan aminotransferase enzyme (TAA)/YUCCA) in land plants. The evolutionary origin of the YUCCA family is currently controversial: YUCCAs are assumed to have emerged via a horizontal gene transfer (HGT) from bacteria to the most recent common ancestor (MRCA) of land plants or to have inherited it from their ancestor, the charophyte algae. To refine YUCCA origin, we performed a phylogenetic analysis of the class B flavoprotein monooxygenases and comparative analysis of the sequences belonging to different families of this protein class. We distinguished a new protein family, named type IIb flavin-containing monooxygenases (FMOs), which comprises homologs of YUCCA from Rhodophyta, Chlorophyta, and Charophyta, land plant proteins, and FMO-E, -F, and -G of the bacterium Rhodococcus jostii RHA1. The type IIb FMOs differ considerably in the sites and domain composition from the other families of class B flavoprotein monooxygenases, YUCCAs included. The phylogenetic analysis also demonstrated that the type IIb FMO clade is not a sibling clade of YUCCAs. We have also identified the bacterial protein group named YUC-like FMOs as the closest to YUCCA homologs. Our results support the hypothesis of the emergence of YUCCA via HGT from bacteria to MRCA of land plants.

The evolutionary origins of auxin transport: what we know and what we need to know

Auxin, represented by indole-3-acetic acid (IAA), has for a long time been studied mainly with respect to the development of land plants, and recent evidence confirms that canonical nuclear auxin signaling is a land plant apomorphy. Increasing sequential and physiological data show that the presence of auxin transport machinery pre-dates the emergence of canonical signaling. In this review, we summarize the present state of knowledge regarding the origins of auxin transport in the green lineage (Viridiplantae), integrating both data from wet lab experiments and sequence evidence on the presence of PIN-FORMED (PIN), PIN-LIKES (PILS), and AUXIN RESISTANT 1/LIKE-AUX1 (AUX1/LAX) homologs. We discuss a high divergence of auxin carrier homologs among algal lineages and emphasize the urgent need for the establishment of good molecular biology models from within the streptophyte green algae. We further postulate and discuss two hypotheses for the ancestral role of auxin in the green lineage. First, auxin was present as a by-product of cell metabolism and the evolution of its transport was stimulated by the need for IAA sequestration and cell detoxification. Second, auxin was primarily a signaling compound, possibly of bacterial origin, and its activity in the pre-plant green algae was a consequence of long-term co-existence with bacteria in shared ecological consortia.

Old Town Roads: routes of auxin biosynthesis across kingdoms

Auxin is an important signaling molecule synthesized in organisms from multiple kingdoms of life, including land plants, green algae, and bacteria. In this review, we highlight the similarities and differences in auxin biosynthesis among these organisms. Tryptophan-dependent routes to IAA are found in land plants, green algae and bacteria. Recent sequencing efforts show that the indole-3-pyruvic acid pathway, one of the primary biosynthetic pathways in land plants, is also found in the green algae. These similarities raise questions about the origin of auxin biosynthesis. Future studies comparing auxin biosynthesis across kingdoms will shed light on its origin and role outside of the plant lineage.

Evolution of Plant Hormone Response Pathways

This review focuses on the evolution of plant hormone signaling pathways. Like the chemical nature of the hormones themselves, the signaling pathways are diverse. Therefore, we focus on a group of hormones whose primary perception mechanism involves an Skp1/Cullin/F-box-type ubiquitin ligase: auxin, jasmonic acid, gibberellic acid, and strigolactone. We begin with a comparison of the core signaling pathways of these four hormones, which have been established through studies conducted in model organisms in the Angiosperms. With the advent of next-generation sequencing and advanced tools for genetic manipulation, the door to understanding the origins of hormone signaling mechanisms in plants beyond these few model systems has opened. For example, in-depth phylogenetic analyses of hormone signaling components are now being complemented by genetic studies in early diverging land plants. Here we discuss recent investigations of how basal land plants make and sense hormones. Finally, we propose connections between the emergence of hormone signaling complexity and major developmental transitions in plant evolution.

Design Principles of Branching Morphogenesis in Filamentous Organisms

The radiation of life on Earth was accompanied by the diversification of multicellular body plans in the eukaryotic kingdoms Animalia, Plantae, Fungi and Chromista. Branching forms are ubiquitous in nature and evolved repeatedly in the above lineages. The developmental and genetic basis of branch formation is well studied in the three-dimensional shoot and root systems of land plants, and in animal organs such as the lung, kidney, mammary gland, vasculature, etc. Notably, recent thought-provoking studies combining experimental analysis and computational modeling of branching patterns in whole animal organs have identified global patterning rules and proposed unifying principles of branching morphogenesis. Filamentous branching forms represent one of the simplest expressions of the multicellular body plan and constitute a key step in the evolution of morphological complexity. Similarities between simple and complex branching forms distantly related in evolution are compelling, raising the question whether shared mechanisms underlie their development. Here, we focus on filamentous branching organisms that represent major study models from three distinct eukaryotic kingdoms, including the moss Physcomitrella patens (Plantae), the brown alga Ectocarpus sp. (Chromista), and the ascomycetes Neurospora crassa and Aspergillus nidulans (Fungi), and bring to light developmental regulatory mechanisms and design principles common to these lineages. Throughout the review we explore how the regulatory mechanisms of branching morphogenesis identified in other models, and in particular animal organs, may inform our thinking on filamentous systems and thereby advance our understanding of the diverse strategies deployed across the eukaryotic tree of life to evolve similar forms.

Large-scale phylogenomic analysis suggests three ancient superclades of the WUSCHEL-RELATED HOMEOBOX transcription factor family in plants

The adaptation of plants to land required multiple morphological innovations. Among these include a variety of lateral organs that are initiated from apical meristems, in which the mantainance of undifferentiated stem cells is regulated by the homeodomain WUSCHEL-RELATED (WOX) transcription factors. Expansion of the WOX gene family has been associated with whole genome duplication (WGD) events and postulated to have been pivotal to the evolution of morphological complexity in land plants. Previous studies have classified the WOX gene family into three superclades (e.g., the ancient clade, the intermediate clade, and the modern clade). In order to improve our understanding of the evolution of the WOX gene family, we surveyed the WOX gene sequences from 38 genomes and 440 transcriptomes spanning the Viridiplantae and Rhodophyta. The WOX phylogeny inferred from 1039 WOX proteins drawn from 267 species with improved support along the backbone of the phylogeny suggests that the plant-specific WOX family contains three ancient superclades, which we term Type 1 (T1WOX, the WOX10/13/14 clade), Type 2 (T2WOX, the WOX8/9 and WOX11/12 clades), and Type 3 (T3WOX, the WUS, WOX1/6, WOX2, WOX3, WOX4 and WOX5/7 clades). Divergence of the T1WOX and T2WOX superclades may predate the diversification of vascular plants. Synteny analysis suggests contribution of WGD to expansion of the WOX family. Promoter analysis finds that the capacity of the WOX genes to be regulated by the auxin and cytokinin signaling pathways may be deeply conserved in the Viridiplantae. This study improves our phylogenetic context for elucidating functional evolution of the WOX gene family, which has likely contributed to the morphological complexity of land plants.

Danger-Associated Peptides Interact with PIN-Dependent Local Auxin Distribution to Inhibit Root Growth in Arabidopsis

Plant elicitor peptides (Peps) are damage/danger-associated molecular patterns that are perceived by the receptor-like kinases, PEPR1 and PEPR2, to enhance innate immunity and to inhibit root growth in Arabidopsis (Arabidopsis thaliana). Here, we show that Arabidopsis Pep1 inhibits root growth in a PEPR2-dependent manner, which is accompanied by swelling epidermal and cortex cells and root hair formation in the transition zone (TZ). These Pep1-induced changes were mimicked by exogenous auxin application and were suppressed in the auxin perception mutants transport inhibitor response1 (tir1) and tir1 afb1 afb2 Pep1-induced auxin accumulation in the TZ region preceded cell expansion in roots. Because local auxin distribution depends on PIN-type auxin transporters, we examined Pep1-PEPR-induced root growth inhibition in several pin mutants and found that pin2 was highly sensitive but pin3 was less sensitive to Pep1. The pin2 pin3 double mutant was as sensitive to Pep1 treatment as wild-type plants. Pep1 reduced the abundance of PIN2 in the plasma membrane through activating endocytosis while increasing PIN3 expression in the TZ, leading to changes in local auxin distribution and inhibiting root growth. These results suggest that Pep-PEPR signaling undergoes crosstalk with auxin accumulation to control cell expansion and differentiation in roots during immune responses.

Control of proliferation in the haploid meristem by CLE peptide signaling in Marchantia polymorpha

The homeostasis of meristems in flowering plants is maintained by cell-to-cell communication via CLE (CLAVATA3/EMBRYO SURROUNDING REGION-related) peptide hormones. In contrast, cell signals that regulate meristem activity remains elusive in bryophytes that maintain apical meristems in the gametophyte (haploid) body and undergo a gametophyte-dominant life cycle. We here show that MpCLE1 confines the proliferative activity of gametophytic meristem and affects the overall size of gametangiophores (reproductive organs) in Marchantia polymorpha, which is in sharp contrast with the meristem-promoting function of its ortholog TDIF/CLE41/CLE44 in Arabidopsis vascular meristems. Expression analysis suggests that MpCLE1 and its receptor gene MpTDR are expressed in distinct patterns across the apical meristem. These data suggest that local CLE peptide signaling may have had a role in regulating cell proliferation in the shoot meristem in the ancestral land plant and acts in both sporophytic and gametophytic meristems of extant plants.

Land plants undergo an alternation of generations where both haploid and diploid phases develop multicellular bodies. Their growth relies on the activity of meristems at the growing tips of their bodies. Here we show a CLE peptide hormone acts as an intercellular signal controlling proliferative activity in the apical meristem of Marchantia polymorpha. Our finding reveals a general association of CLE peptide signaling with meristem homeostasis, a feature that evolved in the ancestral land plant, in both haploid and diploid phases.

The evolution of root branching: increasing the level of plasticity

Plant roots and root systems are indispensable for water and nutrient foraging, and are a major evolutionary achievement for plants to cope with dry land conditions. The ability of roots to branch contributes substantially to their capacity to explore the soil for water and nutrients, and led ~400 million years ago to the successful colonization of land by plants, eventually even in arid regions. During this colonization, different forms of root branching evolved, reinforcing step by step the phenotypic plasticity of the root system. Whereas the lycophytes, the most ancient land plants with roots, only branch at the root tip, ferns are able to form roots laterally in a fixed pattern along the main root. Finally, roots of seed plants show the highest phenotypic plasticity, because lateral roots can possibly, dependent on internal and/or external signals, be produced at almost any position along the main root. The competence to form lateral roots in seed plants is based on the presence of internal cell files with stem cell-like features. Despite the dissimilarities between the different clades, a number of genetic modules seem to be co-opted in order to acquire root branching capacity. In this review, starting from the lateral root pathways in seed plants, we review root branching in the different land plant lineages and discuss the hitherto described genetic modules that contribute to their root branching capacity. We try to obtain insight into how land plants have acquired an increasing root branching plasticity during evolution that contributed to the successful colonization of our planet by seed plants.

Advances in Understanding the Mechanism of Action of the Auxin Permease AUX1

In over 40 years of research on the cellular uptake of auxin it is somewhat chastening that we have elaborated so little on the original kinetic descriptions of auxin uptake by plant cells made by Rubery and Sheldrake in 1974. Every aspect of that seminal work has been investigated in detail, and the uptake activity they measured is now known to be attributed to the AUX1/LAX family of permeases. Recent pharmacological studies have defined the substrate specificity of AUX1, biochemical studies have evaluated its permeability to auxin in plant cell membranes, and rigourous kinetic studies have confirmed the affinity of AUX1 for IAA and synthetic auxins. Advances in genome sequencing have provided a rich resource for informatic analysis of the ancestry of AUX1 and the LAX proteins and, along with models of topology, suggest mechanistic links to families of eukaryotic proton co-transporters for which crystal structures have been presented. The insights gained from all the accumulated research reflect the brilliance of Rubery and Sheldrake’s early work, but recent biochemical analyses are starting to advance further our understanding of this vitally important family of auxin transport proteins.

The PIN-FORMED Auxin Efflux Carriers in Plants

Auxin plays crucial roles in multiple developmental processes, such as embryogenesis, organogenesis, cell determination and division, as well as tropic responses. These processes are finely coordinated by the auxin, which requires the polar distribution of auxin within tissues and cells. The intercellular directionality of auxin flow is closely related to the asymmetric subcellular location of PIN-FORMED (PIN) auxin efflux transporters. All PIN proteins have a conserved structure with a central hydrophilic loop domain, which harbors several phosphosites targeted by a set of protein kinases. The activities of PIN proteins are finely regulated by diverse endogenous and exogenous stimuli at multiple layers-including transcriptional and epigenetic levels, post-transcriptional modifications, subcellular trafficking, as well as PINs' recycling and turnover-to facilitate the developmental processes in an auxin gradient-dependent manner. Here, the recent advances in the structure, evolution, regulation and functions of PIN proteins in plants will be discussed. The information provided by this review will shed new light on the asymmetric auxin-distribution-dependent development processes mediated by PIN transporters in plants.

The Role of the Primary Cell Wall in Plant Morphogenesis

Morphogenesis remains a riddle, wrapped in a mystery, inside an enigma. It remains a formidable problem viewed from many different perspectives of morphology, genetics, and computational modelling. We propose a biochemical reductionist approach that shows how both internal and external physical forces contribute to plant morphogenesis via mechanical stress⁻strain transduction from the primary cell wall tethered to the plasma membrane by a specific arabinogalactan protein (AGP). The resulting stress vector, with direction defined by Hechtian adhesion sites, has a magnitude of a few piconewtons amplified by a hypothetical Hechtian growth oscillator. This paradigm shift involves stress-activated plasma membrane Ca2+ channels and auxin-activated H⁺-ATPase. The proton pump dissociates periplasmic AGP-glycomodules that bind Ca2+. Thus, as the immediate source of cytosolic Ca2+, an AGP-Ca2+ capacitor directs the vectorial exocytosis of cell wall precursors and auxin efflux (PIN) proteins. In toto, these components comprise the Hechtian oscillator and also the gravisensor. Thus, interdependent auxin and Ca2+ morphogen gradients account for the predominance of AGPs. The size and location of a cell surface AGP-Ca2+ capacitor is essential to differentiation and explains AGP correlation with all stages of morphogenetic patterning from embryogenesis to root and shoot. Finally, the evolutionary origins of the Hechtian oscillator in the unicellular Chlorophycean algae reflect the ubiquitous role of chemiosmotic proton pumps that preceded DNA at the dawn of life.

Class C ARFs evolved before the origin of land plants and antagonize differentiation and developmental transitions in Marchantia polymorpha

A plethora of developmental and physiological processes in land plants is influenced by auxin, to a large extent via alterations in gene expression by AUXIN RESPONSE FACTORs (ARFs). The canonical auxin transcriptional response system is a land plant innovation, however, charophycean algae possess orthologues of at least some classes of ARF and AUXIN/INDOLE-3-ACETIC ACID (AUX/IAA) genes, suggesting that elements of the canonical land plant system existed in an ancestral alga. We reconstructed the phylogenetic relationships between streptophyte ARF and AUX/IAA genes and functionally characterized the solitary class C ARF, MpARF3, in Marchantia polymorpha. Phylogenetic analyses indicate that multiple ARF classes, including class C ARFs, existed in an ancestral alga. Loss- and gain-of-function MpARF3 alleles result in pleiotropic effects in the gametophyte, with MpARF3 inhibiting differentiation and developmental transitions in multiple stages of the life cycle. Although loss-of-function Mparf3 and Mpmir160 alleles respond to exogenous auxin treatments, strong miR-resistant MpARF3 alleles are auxin-insensitive, suggesting that class C ARFs act in a context-dependent fashion. We conclude that two modules independently evolved to regulate a pre-existing ARF transcriptional network. Whereas the auxin-TIR1-AUX/IAA pathway evolved to repress class A/B ARF activity, miR160 evolved to repress class C ARFs in a dynamic fashion.

Evolution of the Aux/IAA Gene Family in Hexaploid Wheat

The Aux/IAA (IAA) gene family, involved in the auxin signalling pathway, acts as an important regulator in plant growth and development. In this study, we explored the evolutionary trajectory of the IAA family in common wheat. The results showed ten pairs of paralogs among 34 TaIAA family members. Seven of the pairs might have undergone segmental duplication, and the other three pairs appear to have experienced tandem duplication. Except for TaIAA15-16, these duplication events occurred in the ancestral genomes before the divergence of Triticeae. After that point, two polyploidization events shaped the current TaIAA family consisting of three subgenomic copies. The structure or expression pattern of the TaIAA family begins to differentiate in the hexaploid genome, where TaIAAs in the D genome lost more genes (eight) and protein secondary structures (α1, α3 and β5) than did the other two genomes. Expression analysis showed that six members of the TaIAA family were not expressed, and members such as TaIAA8, 15, 16, 28 and 33 exhibited tissue-specific expression patterns. In addition, three of the ten pairs of paralogs (TaIAA5-12, TaIAA15-16 and TaIAA29-30) showed similar expression patterns, and another five paralog pairs displayed differential expression patterns. Phylogenetic analysis showed that paralog pairs with high rates of evolution (ω > ω ₀), particularly TaIAA15-16 and TaIAA29-30, experienced greater motif loss, with only zero to two interacting IAA proteins. In contrast, most paralogous genes with low ω, such as TaIAA5-12, had more complete motifs and higher degrees of interaction with other family members.

Primitive Auxin Response without TIR1 and Aux/IAA in the Charophyte Alga Klebsormidium nitens

The phytohormone auxin regulates many aspects of growth and development in land plants, but the origin and evolution of auxin signaling and response mechanisms remain largely unknown. Indeed, it remains to be investigated whether auxin-related pathways diverged before the emergence of land plants. To address this knowledge deficit, we analyzed auxin responses in the charophyte alga Klebsormidium nitens NIES-2285, whose ancestor diverged from a green algal ancestor during the evolution of land plants. This strain is the same as Klebsormidium flaccidum NIES-2285, for which the draft genome was sequenced in 2014, and was taxonomically reclassified as K. nitens This genome sequence revealed genes involved in auxin responses. Furthermore, the auxin indole-3-acetic acid (IAA) was detected in cultures of K. nitens, but K. nitens lacks the central regulators of the canonical auxin-signaling pathway found in land plants. Exogenous IAA inhibited cell division and cell elongation in K. nitens Inhibitors of auxin biosynthesis and of polar auxin transport also inhibited cell division and elongation. Moreover, exogenous IAA rapidly induced expression of a LATERAL ORGAN BOUNDARIES-DOMAIN transcription factor. These results suggest that K. nitens has acquired the part of the auxin system that regulates transcription and cell growth without the requirement for the central players that govern auxin signaling in land plants.

Genome-wide identification of SAUR genes in watermelon (Citrullus lanatus)

The early auxin responsive SAUR family is an important gene family in auxin signal transduction. We here present the first report of a genome-wide identification of SAUR genes in watermelon genome. We successfully identified 65 ClaSAURs and provide a genomic framework for future study on these genes. Phylogenetic result revealed a Cucurbitaceae-specific SAUR subfamily and contribute to understanding of the evolutionary pattern of SAUR genes in plants. Quantitative RT-PCR analysis demonstrates the existed expression of 11 randomly selected SAUR genes in watermelon tissues. ClaSAUR36 was highly expressed in fruit, for which further study might bring a new prospective for watermelon fruit development. Moreover, correlation analysis revealed the similar expression profiles of SAUR genes between watermelon and Arabidopsis during shoot organogenesis. This work gives us a new support for the conserved auxin machinery in plants.

Evolution of jasmonate biosynthesis and signaling mechanisms

Jasmonates are phytohormones that modulate a wide spectrum of plant physiological processes, especially defense against herbivores and necrotrophs. The molecular mechanisms of jasmonate biosynthesis and signaling have been well characterized in model plants. In this review, we provide an in-depth analysis and overview of the origin and evolution of the jasmonate biosynthesis and signaling pathways. Furthermore, we discuss the striking parallels between jasmonate and auxin signaling mechanisms, which reveals a common ancestry of these signaling mechanisms. Finally, we highlight the importance of studying jasmonate biosynthesis and signaling in lower plants.

Interaction of chiral herbicides with soil microorganisms, algae and vascular plants

Chiral herbicides are often used in agriculture as racemic mixtures, although studies have shown that the fate and toxicity of herbicide enantiomers to target and non-target plants can be enantioselective and that herbicide toxicity can be mediated by only one enantiomer. If one enantiomer is active against the target plant, the use of enantiomer-rich herbicide mixtures instead of racemic herbicides could decrease the amount of herbicide applied to a crop and the cost of herbicide application, as well as unintended toxic herbicide effects in the environment. Such a change in the management of herbicide applications requires in-depth knowledge and a critical analysis of the fate and effects of herbicide enantiomers in the environment. This review article first synthesizes the current state of knowledge on soil and plant biodegradation of herbicide enantiomers. Second, we discuss our understanding of the biochemical toxicity mechanisms associated with both enantiomers in target and non-target plants gained from state-of-the-art genomic, proteomic and metabolomic tools. Third, we present the emerging view on the "side effects" of herbicides in the root microbiome and their repercussions on target or non-target plant metabolism. Although our review of the literature indicates that the toxicity of herbicide enantiomers is highly variable depending on plant species and herbicides, we found general trends in the enantioselective toxic effects of different herbicides in vascular plants and algae. The present study will be helpful for pesticide risk assessments as well as for the management of applying enriched-enantiomer herbicides.

A sister of PIN1 gene in tomato (Solanum lycopersicum) defines leaf and flower organ initiation patterns by maintaining epidermal auxin flux

The spatiotemporal localization of the plant hormone auxin acts as a positional cue during early leaf and flower organogenesis. One of the main contributors to auxin localization is the auxin efflux carrier PIN-FORMED1 (PIN1). Phylogenetic analysis has revealed that PIN1 genes are split into two sister clades; PIN1 and the relatively uncharacterized Sister-Of-PIN1 (SoPIN1). In this paper we identify entire-2 as a loss-of-function SlSoPIN1a (Solyc10g078370) mutant in Solanum lycopersicum. The entire-2 plants are unable to specify proper leaf initiation leading to a frequent switch from the wild type spiral phyllotactic pattern to distichous and decussate patterns. Leaves in entire-2 are large and less complex and the leaflets display spatial deformities in lamina expansion, vascular development, and margin specification. During sympodial growth in entire-2 the specification of organ position and identity is greatly affected resulting in variable branching patterns on the main sympodial and inflorescence axes. To understand how SlSoPIN1a functions in establishing proper auxin maxima we used the auxin signaling reporter DR5: Venus to visualize differences in auxin localization between entire-2 and wild type. DR5: Venus visualization shows a widening of auxin localization which spreads to subepidermal tissue layers during early leaf and flower organogenesis, showing that SoPIN1 functions to focus auxin signaling to the epidermal layer. The striking spatial deformities observed in entire-2 help provide a mechanistic framework for explaining the function of the SoPIN1 clade in S.lycopersicum.

Q&A: Auxin: the plant molecule that influences almost anything

Auxin is an essential molecule that controls almost every aspect of plant development. Although the core signaling components that control auxin response are well characterized, the precise mechanisms enabling specific responses are not yet fully understood. Considering the significance of auxin in plant growth and its potential applications, deciphering further aspects of its biology is an important and exciting challenge.

Multi-Scale Characean Experimental System: From Electrophysiology of Membrane Transporters to Cell-to-Cell Connectivity, Cytoplasmic Streaming and Auxin Metabolism

The morphology of characean algae could be mistaken for a higher plant: stem-like axes with leaf-like branchlets anchored in the soil by root-like rhizoids. However, all of these structures are made up of giant multinucleate cells separated by multicellular nodal complexes. The excised internodal cells survive long enough for the nodes to give rise to new thallus. The size of the internodes and their thick cytoplasmic layer minimize impalement injury and allow specific micro-electrode placement. The cell structure can be manipulated by centrifugation, perfusion of cell contents or creation of cytoplasmic droplets, allowing access to both vacuolar and cytoplasmic compartments and both sides of the cell membranes. Thousands of electrical measurements on intact or altered cells and cytoplasmic droplets laid down basis to modern plant electrophysiology. Furthermore, the giant internodal cells and whole thalli facilitate research into many other plant properties. As nutrients have to be transported from rhizoids to growing parts of the thallus and hormonal signals need to pass from cell to cell, Characeae possess very fast cytoplasmic streaming. The mechanism was resolved in the characean model. Plasmodesmata between the internodal cells and nodal complexes facilitate transport of ions, nutrients and photosynthates across the nodes. The internal structure was found to be similar to those of higher plants. Recent experiments suggest a strong circadian influence on metabolic pathways producing indole-3-acetic acid (IAA) and serotonin/melatonin. The review will discuss the impact of the characean models arising from fragments of cells, single cells, cell-to-cell transport or whole thalli on understanding of plant evolution and physiology.

Enzyme-Less Growth in Chara and Terrestrial Plants

Enzyme-less chemistry appears to control the growth rate of the green alga Chara corallina. The chemistry occurs in the wall where a calcium pectate cycle determines both the rate of wall enlargement and the rate of pectate deposition into the wall. The process is the first to indicate that a wall polymer can control how a plant cell enlarges after exocytosis releases the polymer to the wall. This raises the question of whether other species use a similar mechanism. Chara is one of the closest relatives of the progenitors of terrestrial plants and during the course of evolution, new wall features evolved while pectate remained one of the most conserved components. In addition, charophytes contain auxin which affects Chara in ways resembling its action in terrestrial plants. Therefore, this review considers whether more recently acquired wall features require different mechanisms to explain cell expansion.

Indole-3-Acetic Acid Is Produced by Emiliania huxleyi Coccolith-Bearing Cells and Triggers a Physiological Response in Bald Cells

Indole-3-acetic acid (IAA) is an auxin produced by terrestrial plants which influences development through a variety of cellular mechanisms, such as altering cell orientation, organ development, fertility, and cell elongation. IAA is also produced by bacterial pathogens and symbionts of plants and algae, allowing them to manipulate growth and development of their host. They do so by either producing excess exogenous IAA or hijacking the IAA biosynthesis pathway of their host. The endogenous production of IAA by algae remains contentious. Using Emiliania huxleyi, a globally abundant marine haptophyte, we investigated the presence and potential role of IAA in algae. Homologs of genes involved in several tryptophan-dependent IAA biosynthesis pathways were identified in E. huxleyi. This suggests that this haptophyte can synthesize IAA using various precursors derived from tryptophan. Addition of L-tryptophan to E. huxleyi stimulated IAA production, which could be detected using Salkowski's reagent and GC × GC-TOFMS in the C cell type (coccolith bearing), but not in the N cell type (bald). Various concentrations of IAA were exogenously added to these two cell types to identify a physiological response in E. huxleyi. The N cell type, which did not produce IAA, was more sensitive to it, showing an increased variation in cell size, membrane permeability, and a corresponding increase in the photosynthetic potential quantum yield of Photosystem II (PSII). A roseobacter (bacteria commonly associated with E. huxleyi) Ruegeria sp. R11, previously shown to produce IAA, was co-cultured with E. huxleyi C and N cells. IAA could not be detected from these co-cultures, and even when stimulated by addition of L-tryptophan, they produced less IAA than axenic C type culture similarly induced. This suggests that IAA plays a novel role signaling between different E. huxleyi cell types, rather than between a bacteria and its algal host.